The present invention relates to the x-ray tube art. It finds particular application in conjunction with high power x-ray tubes for use with CT scanners and the like and will be described with particular reference thereto. It will be appreciated, however, that the invention will also have other applications.
Typically, a high power x-ray tube includes an evacuated envelope or housing which holds cathode filament through which a heating current is passed. This current heats the filament sufficiently that a cloud of electrons is emitted, i.e. thermionic emission occurs. A high potential, on the order of 100-200 kV, is applied between the cathode and an anode which is also located in the evacuated envelope. This potential causes the electrons to flow from the cathode to the anode through the evacuated region in the interior of the evacuated envelope. The electron beam impinges on a small area of the anode or focal spot with sufficient energy that x-rays are generated and extreme heat is produced as a byproduct.
In high energy x-ray tubes, the anode is rotated at a high speed such that the electron beam does not dwell on only the small spot of the anode long enough to cause thermal deformation. The diameter of the anode is sufficiently large that in one rotation of the anode, each spot on the anode that was heated by the electron beam has substantially cooled before returning to be reheated by the electron beam. Larger diameter anodes have larger circumferences, hence provide greater thermal loading. In conventional rotating anode x-ray tubes, the envelope and the cathode remain stationary while the anode rotates inside the envelope. Heat from the anode is dissipated by the thermal radiation through the vacuum to the exterior of the envelope. It is to be appreciated that heat transfer from the anode through the vacuum is limited.
High power x-ray tubes have been proposed in which the anode and vacuum envelope rotate, while the cathode filament inside the envelope remains stationary. This configuration permits a coolant fluid to be circulated through the anode to provide a direct thermal connection between the anode and the exterior of the envelope. See for example, U.S. Pat. Nos. 5,046,186; 4,788,705; 4,878,235; and 2,111,412.
One of the difficulties with this configuration is holding the cathode stationary within the rotating envelope. When the cathode assembly is supported by structures which are rotating with the envelope at a high rate of speed, it tends to rotate with the anode and the envelope.
One technique for holding the cathode stationary is through the use of magnets. One or more stationary magnets are mounted outside of the rotating envelope and couple with a magnetic structure inside the envelope connected with the cathode. One of the problems with these arrangements is that they lack stability and freedom from oscillation. Typically, the magnet assembly is at a relatively small diameter or lever arm. This short lever arm exaggerates the oscillation problem. The magnetic coupling is analogous to a spring. The rotational forces on the cathode tend to move the cathode away from the magnet. The magnet pulls the cathode structure back, but the cathode structure typically overshoots the magnet, going past it in the other direction. The magnet pulls the cathode structure back towards itself again but again there is a tendency to overshoot. In this manner, the cathode tends to oscillate back and forth. Frictional forces transmitted through the bearing or other structures which support the cathode within the envelope supply energy to restart or maintain such oscillations. Such oscillations, of course, oscillate the electron beam, hence the focal spot on the anode where x-rays are generated. This wavering of the focal point of the x-ray beam has detrimental effects, particularly in CT scanners and other high performance x-ray equipment.
The present invention provides a new and improved x-ray tube in which there is a stiff coupling between the electrode and stationary structures on the exterior of the rotating housing.